ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-7-3969-2007Regional pollution potentials of megacities and other major population centersLawrenceM. G.1ButlerT. M.1SteinkampJ.1GurjarB. R.2LelieveldJ.11Max-Planck-Institut für Chemie, P.O. Box 3060, 55020 Mainz, Germany2Indian Institute of Technology Roorkee, Department of Civil Engineering, Roorkee 247667, India2607200771439693987This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.This article is available from http://www.atmos-chem-phys.net/7/3969/2007/acp-7-3969-2007.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/7/3969/2007/acp-7-3969-2007.pdf

Megacities and other major population centers represent large,
concentrated sources of anthropogenic pollutants to the atmosphere,
with consequences for both local air quality and for regional and
global atmospheric chemistry. The tradeoffs between the regional
buildup of pollutants near their sources versus long-range export
depend on meteorological characteristics which vary as a function of
geographical location and season. Both horizontal and vertical
transport contribute to pollutant export, and the overall degree of
export is strongly governed by the lifetimes of pollutants. We
provide a first quantification of these tradeoffs and the main factors
influencing them in terms of "regional pollution potentials",
metrics based on simulations of representative tracers using the 3-D
global model MATCH (Model of Atmospheric Transport and Chemistry).
The tracers have three different lifetimes (1, 10, and 100 days) and
are emitted from 36 continental large point sources. Several key
features of the export characteristics emerge. For instance,
long-range near-surface pollutant export is generally strongest in the
middle and high latitudes, especially for source locations in Eurasia,
for which 17&ndash;34% of a tracer with a 10-day lifetime is exported
beyond 1000 km and still remains below 1 km altitude. On the other
hand, pollutant export to the upper troposphere is greatest in the
tropics, due to transport by deep convection, and for six source
locations, more than 50% of the total mass of the 10-day lifetime
tracer is found above 5 km altitude. Furthermore, not only are
there order of magnitude interregional differences, such as between
low and high latitudes, but also often substantial intraregional
differences, which we discuss in light of the regional meteorological
characteristics. We also contrast the roles of horizontal dilution
and vertical mixing in reducing the pollution buildup in the regions
including and surrounding the sources. For some regions such as
Eurasia, dilution due to long-range horizontal transport governs the
local and regional pollution buildup; however, on a global basis,
differences in vertical mixing are dominant in determining the
pollution buildup both around and further downwind of the source
locations.